1. Field of the Invention
The present invention relates to an image data transmission apparatus for transmitting image data, and more particularly, to an image data transmission apparatus that reads a document placed on the apparatus and transmits the image data of the document with a desired line density (resolution).
2. Description of the Related Art
Facsimile equipment is now widely used as an image data transmission apparatus for reading a given document and transmitting the image data of the document. Such facsimile equipment can transmit image data with various line densities such as those given in normal mode (8 pel/mm.times.3.85 line/mm), fine mode (8 pel/mm.times.7.7 line/mm), super fine mode (8 pel/mm.times.15.4 line/mm), and ultra fine mode (16 pel/mm.times.15.4 line/mm) wherein the line density in the principal scanning direction can be set to 8 pel/mm or 16 pel/mm, and the line density in the secondary scanning direction can be set to 3.85 line/mm, 7.7 line/mm or 15.4 line/mm.
Some known facsimile apparatuses have the capability of reading image data from a document of A3 size (297 mm.times.420 mm) in such a manner that the principal scanning is performed in a direction along the shorter sides of the document. In this case, when a A3-size document is placed on facsimile equipment, the principal scanning is normally performed along the shorter sides of the document and the line density in the principal scanning direction is set to 8 pel/mm or 16 pel/mm, whereas the secondary scanning is performed along the longer sides with the line density of 3.85 line/mm, 7.7 line/mm or 15.4 line/mm. The image data acquired by the above scanning operation is then transmitted. However, if an A4-size (210 mm.times.297 mm) document is placed on the facsimile apparatus in such a manner that the principal scanning is performed along the longer sides of the document (hereafter such a document position will be referred to as a "horizontal position", and 90.degree.-rotated document position will be referred to as a "vertical position"), the line density in the principal scanning direction along the longer sides becomes equal to the line density which is obtained in the secondary scanning direction for an A3-size document, and the line density in the secondary scanning direction for the A4-size document becomes equal to the line density in the principal scanning direction for the A3-size document. In this case, the image data obtained in the scanning operation is transmitted after rotating the image data by an amount of 90.degree. so that the transmitted image becomes similar to that obtained when the A4-size document is placed on the facsimile equipment in such a manner that the principal scanning is performed along the shorter sides.
However, in the above-described conventional facsimile equipment, when an A4-size document is placed in the horizontal position on the facsimile equipment so that the principal scanning is performed along the longer sides, the image data is transmitted after rotating the image by 90.degree. regardless of the capability of a facsimile apparatus at a receiving end. This means that the image is rotated by 90.degree. before transmission even if the receiving facsimile apparatus has the capability of printing received image data on A3-size paper or the capability of printing image data on A4-size paper in such a manner that the principal printing is performed along the longer sides of the A4-size paper. An image of a horizontally-positioned A4-size document is always converted into a vertical-position image, although a high speed receiving and printing operation will be achieved if a sending apparatus transmits a horizontal-position image without rotation and a receiving apparatus prints the received image on horizontally-positioned paper.